Welding is an established technique for joining components made of plastic. For this purpose, various options are used for providing energy input. Examples are friction welding or ultrasonic welding.
In particular for use for automotive exteriors, due to the possibilities offered by the new material properties in injection molding the current common wall thicknesses during processing are in the range of ≧3.0 mm. The standard type of material used is talc-reinforced PP for cost reasons.
Costs are reduced and component weight is decreased due to the thin wall thickness. However, since compared to other commercial plastics, PP in particular is very susceptible to creep under load and has a low modulus of elasticity, but at the same time the requirements imposed on component welding for retention forces of >200 N at each weld point on the component joint remain unchanged, it is often not possible to join the components in such a way that the surface relevant to the appearance remains free of defects. Two influencing factors hinder mark-free welding:
1.) Process-Related Defects                The energy introduced for the melting and intimate connection of the parting plane acts through the component in such a way that the surface is deformed due to the difference in shrinkage during cooling of the component, resulting in formation of a defect.        
2.) Load-Related Defects                The forces introduced into the component joint in a punctiform manner during the welding result in visible deformations at the surface when the component thickness and the modulus of elasticity of the surface are excessively low. The surface gives way under the load of the introduced stresses, resulting in formation of a visible defect.        
One development for preventing this effect is torsional ultrasonic welding, as described in EP 1 930 148 A1 and currently unpublished DE 10 2009 011273 A1 The upper component situated beneath the sonotrode is set in rotatory vibration in the ultrasonic frequency range. The vibration is transmitted through the upper component, and is converted to heat directly in the parting plane between the upper and lower components. Although the upper component is heated due to its internal damping, it is not melted. On the one hand this reduces the heat acting on the lower component, and on the other hand the effective joining surface for the introduction of force is changed from a punctiform connection to a linear and/or planar connection. As a result, mark-free connections for polyolefinic materials are possible, even for a wall thickness less than 3.0 mm. The occurrence of defects may be prevented in this way.
In the present description, the terms “upper component” and “mounting part,” and “lower component” and “component,” respectively refer to the same objects. The sonotrode is placed on the mounting part, i.e., the upper component, in order to weld the component situated beneath the mounting part to the mounting part.
The ultrasonic welding technique known as torsional welding requires a high contact pressure. In particular when the surface is slightly soiled from preceding work steps, the required temperature of the weld metal is often not reached, and instead the weld metal is deformed and an intimate connection is prevented. In addition, due to the high contact pressure the liquefied material is pushed from the region of the weld point. A cold weld point with poor adhesion is the result.